Fulvic acid studies: evidence for a polycarboxylate co-ordination mode at soil pH

The reactions of two fulvic acid samples with Cu²⁺ and Cd²⁺ have been studied by potentiometric titration (pH and ion selective electrode). The metal-uptake curves for samples initially rendered ‘metal-free’ by a precipitation-solvent extraction technique resemble those for citric and malic acids at soil pH (4–6.5). The presence of aliphatic and aromatic components was established by ¹³C nmr spectroscopy; the presence of phenolic components was inferred from pH titrations. It is shown that a polycarboxylate component (simulated by citrate) would mask reactions of phenolic components with metals in weakly acidic and neutral media. The mode of coordination between fulvic acid and metals will vary with pH and with the metal: fulvic acid ratio.